The recent development of the microstructured optical fiber, in which a high index core region is surrounded by cladding having a mix of silica and air, offers new fiber properties by virtue of the large refractive-index contrast that exists between glass and air. A cladding structure may have a spatially uniform average refractive index that can be adjusted to meet a desired relationship with the core index. FIG. 1 illustrates a prior art microstructured optical fiber in cross section. In the optical fiber 1b, a plurality of holes 3b are arranged in silica glass 2b. The diameters of the holes 3b are substantially uniform in the cross section and the centers of the holes 3b substantially coincide with the lattice points 4b of a hexagonal lattice. With substantially periodically arranged sub medium regions in the cladding, the core region can be realized by substituting one or more sub medium regions with main medium (silica glass, for example). A lattice point 41b in the center of the fiber has no corresponding hole, so that the center of the fiber has a higher average refractive index than that of the surrounding region. As a result, a lightwave is localized in the center of the fiber and guided over the fiber.
It is difficult to realize a high yield of production and a low transmission loss in the production of microstructured optical fibers with zero or negative chromatic dispersion, zero or negative chromatic dispersion slope, and/or small effective core area. For realizing such characteristics in microstructured optical fibers, air holes with small diameters and accurate control of the dimension and arrangement of the air holes are necessary. However, decrease in the hole diameter causes increase in the surface tension at the surfaces of the holes during fiber drawing. The increase in surface tension causes excess shrinking of the holes resulting in increase in the uncontrollability of the optical characteristics of the drawn optical fiber. Although influence of surface tension can be decreased by lowering the drawing temperature, the drawing tension increases by decrease in drawing temperature, resulting in increased occurrence of fiber breaking during drawing, increase in transmission loss, and degradation in durability against UV radiation.